If a throttle cable breaks...

I had a carb flange boot fail on the ground while taxiing and the feel and sound was ugly, would not want to experience losing one side of the engine in flight. The shaking was so violent that it seemed that it would not take long for the exhaust system, the working carburetor and other accesories to fall off or be damaged. After that experience everytime I remove the engine cowling I give the throttle system and carb flange mounts extra attention.

Check the Rotax installation manual...  I believe it calls for springs on BOTH carbs to pull the throttle arm to fully open in the event your throttle cable breaks.  I know that's how mine is installed, and its how I've seen it on every Rotax instalaltion I've seen thus far.  It's also shown that way in the "carb synch" video on this site.

I think there is logic in that configuration:  If the throttle breaks while you're in flight, full power is NOT an emergency.  You can continue to fly to your next available airport, and land the plane by switching the ignition off to kill the power.  But if those springs were reversed, and pulled the throttle arm to the idle position immediately, it would be effectively the same as an engine failure – you're going to have to land immediately – somewhere within gliding distance.  

Likewise, with both springs installed correctly, a throttle cable break during takeoff would become a relative "non-event" – you would likely not even notice until you tried to reduce power.

Having only one spring pulling one throttle to max power setting would be a very bad thing.  I suspect that someone forgot to move the spring back to the "increase" side following your last carb synch maintenance...

Hi James, yes, both carbs have a spring. There are numerous installation configuration choices in the homebuilder world to connect the 912 carburetors to the pilot’s hand in the cockpit.  In my aircraft, which is the kit factory’s recommended dual throttle setup, it is possible for a cable on one side of the engine to break (or have a cable clamp fail, etc.) especially where the cable connects with the carburetor such that only that carburetor would be pulled to wide open. There’s no single cable that connects both carbs to the Bowden cables that route into the cockpit. I’d post up a photo to make it easier to visualize but this site won’t allow photos to be pasted in directly from ones computer, only links to a website with a photo are permitted.

In that case, if/when a cable breaks, one carb will be pulled to wide-open, and you'll get roughness.  The solution would be to treat it as I described – go to full throttle, fly to a safe landing area, and shut down the engine for a deadstick landing.  (I would wait to kill the engine until I was on short final, with the landing 100% assured...)